Throughout this application, the following terms are meant to convey the accompanying definitions.
C-arm: means an intra-operative x-ray (fluoroscopy)
C-arm pose: means the three-dimensional (3D) location and orientation of the x-ray source (intensifier)
C-arm pose tracking: means to estimate the C-arm pose from one pose to another pose (the same as pose recovery). A transformation matrix encodes 3 translations and three rotations to determine a second pose from a first pose.
Bronchoscopy is a real-time modality for inspecting the internal walls of the bronchial airways and conducting subsequent diagnostic or therapeutic interventions using an endoscope. During the bronchoscopy procedure, a C-arm can be used for real-time monitoring of the location of the endoscope. However, due to the low image contrast of x-ray images, both a lesion and the airways are difficult to see. This hinders the utility of the X-ray fluoroscopy as a real-time image guidance tool in assisting bronchoscopic interventions.
One solution to the low contrast problem is to fuse a three-dimensional (3D) computed tomography (CT) image volume with two-dimensional X-ray images, and overlay a lesion to the X-ray images. The 3D image volume is fused to the 2D x-ray images by registration between the 2D images and 3D image volume. The fused visualization provides an intra-operative guidance so that one can precisely localize the lesion and identify its spatial relationship to the endoscope.
2D-3D registration between x-ray images and CT volume can image-based or fiducial-based. Image-based registration methods, which use segmentation, have comparatively small capture range and slow convergence rate. Moreover, the computational overhead for image-based registration is intensive. Also, when employing the features in the image, the result is dependent upon the accuracy of the segmentation and the characteristics of the features.
To facilitate an image-based registration method, a good initialization pose of the C-arm has to be provided to reduce searching space in the optimization scheme. However, when the C-arm moves to a different pose intra-operatively, the initialization is no longer available in the new C-arm pose. Thus, real-time fusion of 2D x-ray and 3D image volume data is not always feasible.
Fiducial-based registration addresses the problem of pose recovery after intra-operative C-arm movement. In fiducial-based registration, radiopaque devices, called fiducials are placed externally on a patient's body. The fiducials are captured in both the x-ray frame prior to intra-operative C-arm movement and the x-ray frame after intra-operative C-arm movement. Because the fiducials must be captured in both frames, they limit the size of the volume available for intra-operative guidance.
The x-ray images of the fiducials are two-dimensional in nature. If the 3D configuration of the external fiducials is unknown, for registration to happen, epipolar geometry must be used to reconstruct the 3D location of the fiducials in x-ray space. Then, the fiducials in x-ray space can be registered to fiducials in the CT space. Epipolar geometry takes the two known C-arm poses as input.
Thus, both image-based and fiducial-based 2D/3D registration require a fast and accurate C-arm pose recovery method. Unfortunately, only a very few hospitals have the expensive, high-end mobile C-arms that have the capacity to estimate or track X-ray poses. Most clinical C-arms currently used in hospitals do not provide pose tracking capability.